Sensory impact of sunburn in white wine and mitigation of climateinduced off-flavours by defoliation and application of reflecting particles on grapes

Abstract

Climate change is a great environmental challenge with large impact on the Wine and sprakling wine industry. Heat waves and dryness cause frequent sunburn damage in white grapes. This leads not only to severe yield loss but also to sensory changes of white wine leading to climate-induced off-flavours. This study aims to develop viticultural and oenological strategies to minimize the sunburn damage to identify the molecular substance, which triggers the Off-Flavours and to prevent them. In 2020 defoliation measures, the application of kaolin and calcium carbonate suspensions on the grapes and a combination of both measures were investigated in respect to mitigate sunburn damage in Riesling vineyards in Germany. Defoliation was done at the early flowering state and late véraison. Early defoliation leads to longer sun exposition of the grapes, triggering molecular protection mechanisms against sunburn. Application of reflecting particles shall protect the grapes from solar radiation during heat periods.
Seven treatments were replicated in three field trials. Grapes of each field trial were separately vinified using the same protocol. The resulting wines were analysed by descriptive analysis and temporal dominance of sensations (TDS) with a trained panel. Additionally, we analysed the aroma composition of the wines using an established SIDA HS-SPME-GC-MS method.
The early sun exposition treatment with partial defoliation of 75 % of the grape zone during flowering and second defoliation of 75 % at véraison creates wines with less fruity aroma, but with a smoky taint (4-vinylguajacol) and the atypical aging note (ATA), reminiscent of acacia blossom and fusel alcohols. Application of kaolin or calcium carbonate slightly mitigated these unpleasant effects and even increased fruity aroma (fruity esters) and a more sweet taste impression. The late defoliation treatment (100 % at véraison) reduced both the green notes of the control and the smoky and ATA nuances, occurring with an early partial defoliation. However, these wines exhibited slight petrol off-flavour, which was corrobated by increased levels of TDN (1,1,6-Trimethyl-1,2-dihydronaphthalene) and vitispirane. Vice versa, early defoliation enhanced formation of floral compounds such as linalool, 2-phenylethanol and ß-damascenon. Wines varied substantially in the sour taste, which was not linked to pH. The TDS results additionally revealed for early defoliated treatments and those receiving kaolin and calcium carbonate protection a stronger fruity and sweet dominance in the first 10 seconds, while late defoliated treatments resulted in a more dominating and lasting sourness.
In conclusion, early defoliation with protecting particles not only favours acclimatisation of grapes to sun exposure leading to less sunburn, but also produces more fruity and less smoky, petrol flavoured wines